In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. User equipments are served in the cells by the respective radio base station and are communicating with the respective radio base station. The user equipments transmit data over an air interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data to the user equipments in downlink (DL) transmissions. The user equipments and radio base stations are organized to communicate using radio frames comprising subframes.
In, for example, LTE a size of radio frames or subframes in a time domain may be expressed as a number of time units, Ts, where Ts=1/(15000×2048) seconds. In the time domain, LTE DL transmissions are organized into radio frames with a time duration of Tf, where Tf=307200×Ts=10 ms. Each radio frame comprises ten equally-sized subframes, denoted as #0-#9, of respective time length of 30720×Ts=1 ms.
A downlink radio frame at the user equipment is synchronised to a radio network clock broadcasted from the radio base station within the cell. Transmission of an uplink radio frame from the user equipment starts (NTA+NTA offset)×Ts seconds before the start of the corresponding downlink radio frame at the user equipment. NTA is defined as a timing advance value, defining a timing for the transmission of uplink radio frame before the time of the corresponding downlink radio frame at the user equipment. NTA is expressed in units of Ts and 0≦NTA≦20512, and transmitted from the radio base station to the user equipment in a Timing Advance Command. NTA offset is defined as fixed timing advanced offset expressed in units of Ts. NTA offset=0 for a radio frame structure according to Frequency Division Duplex (FDD) operation and is a fixed value set at the user equipment. Thus, a user equipment operating in FDD mode, where NTA offset=0, is able to start a transmission of an uplink radio frame up to 20512Ts, or ˜668 μs, before the reception of the corresponding downlink radio frame.
The purpose of the above function is to enable the signals, that is, uplink transmissions in radio frames, from user equipments at varying distances from the radio base station to have their uplink transmission in radio frames aligned in time at the radio base station receiver. User equipments at a long distance from the radio base station will have to advance their uplink transmissions with the amount corresponding to two times the propagation delay from the user equipment transmitter to the user equipment receiver. 668 μs would thus correspond to a radio base station to user equipment distance of about 100 km. Each active user equipment has its timing monitored and tracked by the radio base station. When needed, the radio base station sends Timing Alignment Commands (TAC) to the user equipment, to order the user equipment to change its uplink timing of the uplink transmission. Typically this is needed when the user equipment is moving towards or away from the radio base station.
Due to the requirements on the radio base station to provide fast feedback on user equipment transmissions, so called Hybrid Automatic Repeat Request (HARQ) feedback, a time budget for processing in the radio base station is restricted. A requirement of HARQ in the uplink is that a retransmission needs to be performed within 8 ms from the original uplink transmission. Within this retransmission time the user equipment should perform the original uplink transmission and the radio base station should receive and handle the data. Furthermore, the radio base station should transmit ACK/NAK and the user equipment should receive and handle the data before performing retransmission. This leads to that the radio base station has typically about 3 ms as the time budget for processing the data. This time budget should be enough to cover reception, scheduling and transmission.
On the user equipment side, a time budget for processing is in the range 2,332 ms to 3 ms, depending on the distance from the radio base station.
The time budget for processing in the radio base station of 3 ms is enough when all the Layer 1 and Layer 2 algorithms during communication are executed. Layer 1 is the physical layer and layer 2 is the data link layer. The time budget for processing is today fixed, which leads to a limited performance of the radio base station.
Time alignment is disclosed in document “UL/DL Timing Alignment for Extended Cell Radius” Alcatel-Lucent Alcatel-Lucent Shanghai Bell Publication data: 3GPP Draft; R1-094602, 20091109 3rd Generation Partnership Project (3GPP), Mobile Competence Centre; 650, route des Lucioles; F-06921 Sophia-Antipolis Cedex; France; Nr: Jeju; 20091109 XP 050389013. Processing time is mentioned in “Analysis of impact of cell range extension,” Alcatel-Lucent Alcatel-Lucent Shanghai Bell Publication data: 3GPP Draft; R1-094601, 20091109 3rd Generation Partnership Project (3GPP), Mobile Competence Centre; 650, route des Lucioles; F-06921 Sophia-Antipolis Cedex; France; Nr: Jeju; 20091109; XP 050389012.